1. Field of the Invention
The present invention relates to optical information recording/reproducing methods for recording multilevel information by using three or more levels of information pits and/or for reproducing the recorded multilevel information. Particularly, the invention relates to an optical information recording/reproducing method that corrects for effects of intersymbol interference and/or nonlinearity on a reproduced signal obtained from an optical information medium.
2. Description of Related Art
The optical memory industry is expanding in recent years with the development of read-only compact disks (CDs) and digital versatile disks (DVDs), write-once memories made of thin metal films or dye recording materials, and ultimately, rewritable memories made of magneto-optical materials or phase change materials. Also, optical memories now have a wider range of applications from consumer products to external memories of computers.
Research and development are underway to enhance the density of recording capacity. One of techniques for reducing the size of a light spot for recording and reproducing information is to use a blue-violet light source (wavelength: 405 nm) rather than a red light source (wavelength: 650 nm). Another technique for reducing the size of the light spot is to increase the numerical aperture (NA) of an objective lens from 0.6 or 0.65 to 0.85. At the same time, multiple-valued recording/reproducing techniques that achieve higher efficiency with the same light spot size as before have been proposed.
For example, the inventor of the present invention has proposed a technique for recording and reproducing multilevel information in Japanese Patent Laid-Open No. 5-128530. The technique involves a recording method for recording multilevel information on an information track of an optical information recording medium by using a combination of the width of an information pit in the track direction and the amount of shift of the information pit in that direction with respect to a light spot for reproduction. It also involves a reproducing method for reproducing the recorded multilevel information on the basis of a correlation between a detected signal learned in advance and a detected signal obtained from the light spot.
Another example of the multiple-valued recording/reproducing techniques has been presented at the International Symposium on Optical Memory (ISOM) 2003, an international academic conference for research in the field of optical disks (“Write-once Disks for Multi-level Optical Recording”, Conference Papers Fr-Po-04). The presentation is about eight-level multiple-valued recording/reproduction using a blue-violet light source (405 nm) and an optical system with an NA of 0.65, and setting the width of a region (hereinafter referred to as “cell”), which is a virtually provided recording area on an optical disk having a track pitch of 0.46 μm, and in which one information pit is recorded to 0.26 μm in the track direction.
Also, the applicant of the present invention has proposed a technique using a blue-violet light source (405 nm) and an optical system with an NA of 0.85 to reduce the size of a light spot, adapted to the multiple-valued method presented at the ISOM 2003, and thus achieving a recording density of as high as 30 Gbit/inch2.
As illustrated in FIG. 17, for the selection of an information pit of eight levels, the width of a cell in the track direction (indicated by “A” in FIG. 17) is divided into 16 equal parts (16 channel bits), for example, with Level 0 being no information pit recorded, Level 1 corresponding to the width of two channel bits, Level 2 corresponding to the width of four channel bits, Level 3 corresponding to the width of six channel bits, Level 4 corresponding to the width of eight channel bits, Level 5 corresponding to the width of 10 channel bits, Level 6 corresponding to the width of 12 channel bits, and Level 7 corresponding to the width of 14 channel bits.
FIG. 18 illustrates a relationship between a light spot and information pits recorded at random on a track of an optical disk.
To increase memory capacity, the size of a cell needs to be reduced. As a result of the reduction of the cell size, as illustrated in FIG. 18, information pits 12 for two or three cells are included in a light spot 13. In FIG. 18, arrow A indicates the direction of a track 11 on the optical disk. Regions separated by dashed lines represent respective cells that are virtually provided.
Here, each cell is 0.2 μm in width while the light spot 13 is about 0.405 μm in diameter. With these dimensions, a surface density of about 1.5 times higher than a surface density of about 19.5 Gbit/inch2 for a known two-valued method (e.g., 1-7 PP modulation, 2T=139 nm) can be achieved.
Next, the results of optical simulations carried out for observing a reproduced signal when the multiple-valued method is used will be described.
FIG. 19 illustrates parameters used in the optical simulations. The track pitch of an optical disk is 0.32 μm, the size of a light spot is 0.405 μm (wavelength: 405 nm, NA of an objective lens: 0.85), and the size of a cell is 0.2 μm. Different shapes as in FIG. 20 are assigned to the respective levels of an information pit illustrated in FIG. 17.
FIG. 21 shows a result of calculations of reproduced signals (the amount of reflected light) obtained by sequentially creating and assigning the combinations of three levels (the total number of combinations is 8×8×8=512) to a set of three consecutive cells, each cell being assigned one of the above-described eight levels, and moving the light spot from the first cell (preceding cell) through the second cell (center cell) to the third cell (subsequent cell).
Referring to FIG. 21, eight combinations of the levels of the three consecutive cells, (0,1,6) through (7,1,6), are shown for exemplary purposes. All cells other than these three are assigned Level 0.
In FIG. 21, the location of each of the three solid lines indicates the intensity of a reproduced signal (cell center value) when the light spot is located at the center of each cell, and the location of each of the two dashed lines indicates the intensity of a reproduced signal (cell boundary value) when the light spot is located at the boundary of one cell and its subsequent cell.
As can be seen, under the parameters described above, the cell center value of the center cell in every combination corresponds to Level 1. However, since the levels of the first cell (preceding cell) range from Level 0 to Level 7, the cell center value of the second cell (center cell) is varied accordingly. This is due to an effect of intersymbol interference. On the other hand, since the level of the second cell (center cell) is Level 1 in every combination, the cell center value of the third cell (subsequent cell) remains substantially the same regardless of the level of the first cell (preceding cell) at the left end. In other words, intersymbol interference originating from one cell has a certain effect on the cell center values of adjacent cells on both left and right sides, but has only a negligible effect on the cell center values of other distant cells. This can be intuitively understood from FIG. 18 where the light spot covers one cell and its two adjacent cells only.
FIG. 22, where the horizontal axis represents the level of the center cell, illustrates the distribution of the amplitude (normalized by the reflectance of marked and unmarked portions) of the reproduced signal for all combinations of levels recorded in three consecutive cells. In FIG. 22, distributions A through H correspond to Level 0 through Level 7, respectively.
As can be seen from FIG. 22, since the distribution of the reproduced signal for one level overlaps with those for adjacent levels, it is difficult to identify them separately with a fixed threshold. To enhance the separation of the distributions of amplitude, reproduced signals are generally subjected to signal processing, such as waveform equalization. For example, three-tap waveform equalization as illustrated in FIG. 23 is performed. In FIG. 23, T represents a time period during which a light spot moves from the center of one cell to that of its adjacent cell and “a” represents an equalization coefficient. Here, the waveform equalization is performed using the equalization coefficient “a” determined by substituting V1=0.237 into a=−V1/(1+V1), where V1 represents an amplitude value for a cell adjacent to a cell corresponding to an isolated waveform with an amplitude of 1.
FIG. 24 shows a result of this waveform equalization. Distributions A′ through H′ correspond to Level 0 through Level 7, respectively. As can be seen, these distributions can be separated from each other with their respective fixed thresholds.
However, this is the result of an ideal simulation and in practice, each distribution may extend due to media noise, recording noise, system noise, or the like. This may lead to reproduction errors and reduced reproduction margins. Therefore, correction processing that can further enhance the separation of the distributions of reproduced signals is necessary.